British police are racing to identify the substance suspected of striking down a former Russian double agent convicted of treason in Moscow for betraying dozens of spies to British intelligence.
Britain's top counter-terrorism officer, Mark Rowley, said investigators needed to be "alive to the fact of state threats" after Sergei Skripal, once a colonel in Russia's GRU military intelligence service, was taken ill.
The 66-year-old former spy and his 33-year-old daughter, Yulia, were found on Sunday unconscious on a bench outside a shopping centre in the English city of Salisbury after exposure to what police said was an unknown substance.
Both were critically ill in intensive care.
Skripal, who passed the identity of dozens of spies to the MI6 foreign intelligence agency, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.
While the British authorities said there was no known risk to the public, police sealed off the area where the former spy was found and a pizza restaurant called Zizzi in the centre of Salisbury. Some investigators wore yellow chemical suits.
"We're speaking to witnesses, we're taking forensic samples at the scene, we're doing toxicology work and that will help us to get to an answer," Rowley told BBC radio. He said counter-terrorism police were assisting the investigation.
"We have to remember: Russian exiles aren't immortal, they do all die and there can be a tendency to conspiracy theories. But likewise we have to be alive to the fact of state threats," he said, pointing to the murder of Alexander Litvinenko.
A British inquiry said President Vladimir Putin probably approved the 2006 murder of ex-KGB agent Litvinenko with radioactive polonium-210 in London. The Kremlin has repeatedly denied any involvement in the killing of Litvinenko.
Litvinenko, 43, an outspoken critic of Putin who fled Russia for Britain six years before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London's Millennium Hotel.
It took some time for British doctors to discern the cause of Litvinenko's illness.
British police did not release the names of those who were being treated but two sources close to the investigation told Reuters that the critically ill man was Skripal. It was unclear what the substance was, they said.
The BBC, which first reported Skripal's name, said his daughter Yulia was the 33-year-old woman found beside him.
The Kremlin said it was ready to co-operate if Britain asks it for help investigating the incident with Skripal.
Skripal was arrested in 2004 by Russia's Federal Security Service (FSB) on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006 after a secret trial.
Skripal, who was at the time shown wearing a track suit in a cage in court during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bank account, Russian media said at the time.
But he was pardoned in 2010 by then-President Dmitry Medvedev as part of a swap to bring 10 Russian agents held in the United States back to Moscow.
Australian Associated Press